It is well known that materials such as lithium niobate (LiNbO3) have an electro-optic effect by which a refractive index of a light is changed by applying an electric field.
A lithium niobate optical modulation device of a traveling wave electrode type (LN optical modulation device) configured by forming an optical waveguide and a traveling-wave electrode on a substrate made of such a material (hereinafter, abbreviated as an LN substrate) is used in high volume optical transmission systems of 2.5 Gbit/s and 10 Gbit/s because of its good chirping characteristics.
In recent years, as this LN optical modulation device is to be further applied to an optical transmission system having a very high capacity of 40 Gbit/s, the device needs to be further developed in light of its importance in the field of optical communications.
FIG. 18 is a perspective view showing a schematic configuration of a typical optical modulation device using lithium niobate (LiNbO3).
Further, FIG. 19 is a sectional view of the optical modulation device in FIG. 18 taken along the line 19—19.
In FIG. 18 and FIG. 19, an optical waveguide 2 is formed from one end of an LN substrate 1 to the other end thereof in contact with an upper surface of the LN substrate 1 in the z-cut state where the lithium niobate (LiNbO3) is cut in the z-surface direction of a crystal surface.
The optical waveguide 2 is branched into two optical waveguides 2a and 2b at the midpoint of the LN substrate 1, which are brought together again in the vicinity of the other end.
A buffer layer 5 commonly covering the upper surfaces of these optical waveguides 2a and 2b and the upper surface of the LN substrate 1 is formed.
A center electrode 3 is formed at a position opposite to the one optical waveguide 2a in the upper surface of the buffer layer 5.
Further, in the upper surface of the buffer layer 5, ground electrodes 4a and 4b are formed, respectively, at the position not opposite to the one optical waveguide 2a and at the position opposite to the other optical waveguide 2b so as to sandwich the center electrode 3.
The discussion here can be applied to any traveling-wave electrode in any form, but as one example, it is assumed that a coplanar waveguide (CPW) having one center electrode 3 and two ground electrodes 4a, 4b is employed.
Furthermore, the optical waveguides 2, 2a, 2b are so-called thermal diffusion optical waveguides formed by, after depositing a metal titanium (Ti) in the thickness from several tens nm to 100 nm or more, patterning it in the width in the order of 6 to 8 μm, and further thermally diffusing it at a temperature of about 1000° C.
In the optical modulation device shown in FIG. 18 and FIG. 19, the optical waveguides 2, 2a, and 2b as a Mach-Zehnder interferometer are configured.
In addition, a one-linear optical waveguide may be employed instead of the Mach-Zehnder interferometer in the case of a phase modulation device.
The buffer layer 5 is deposited between the traveling-wave electrode formed of the center electrode 3 and the ground electrodes 4a, 4b and the LN substrate 1 in order to restrict an absorption loss which a light guided through the optical waveguides 2a and 2b receives from the metal (Au is generally employed) which is the traveling-wave electrode (center electrode 3, ground electrodes 4a, 4b).
The buffer layer 5 is generally made of SiO2 which is as thick as about 1 μm.
The buffer layer 5 is used for reducing a microwave equivalent refractive index of an electric signal guided through the traveling-wave electrode formed of the center electrode 3 and the ground electrodes 4a, 4b (or microwave equivalent refractive index of the traveling-wave electrode) to be made closer to an equivalent refractive index of the light guided through the optical waveguides 2a and 2b (or equivalent refractive index of the optical waveguide), and making a characteristic impedance closer to 50 Ω, as well as for restricting the absorption loss.
FIG. 20 is a diagram shown for explaining operations of the optical modulation device having such a structure.
In other words, FIG. 20 shows a desirable distribution of an electric line of force 7a in the case where a voltage is applied between the center electrode 3 and the ground electrodes 4a, 4b of the traveling-wave electrode.
As can be understood from FIG. 20, since the orientation of the electric line of force 7a across the two optical waveguides 2a and 2b is reverse to the orientation of both the optical waveguides, a phase of the light guided through the two optical waveguides 2a and 2b is shifted by 180° (π) in the optical waveguide of the Mach-Zehnder interferometer so that the OFF state of the light can be realized.
However, even an optical modulation device having a sectional structure shown in FIG. 19 still has disadvantages, as described below.
Since the LN substrate 1 has a pyroelectric effect, as shown in FIG. 21, when the temperature of the LN substrate 1 is changed, a charge 9a is induced on the surface thereof.
However, since the buffer layer 5 made of SiO2 does not have conductivity, in the center electrode 3 and the ground electrodes 4a, 4b of the traveling-wave electrode, a charge 9b having a polarity opposite to the polarity of the charge 9a induced on the surface of the LN substrate 1 is induced on the surface opposite the LN substrate 1 through an external circuit.
As a result, an electric line of force 7b is generated between the charge 9a induced on the LN substrate 1 and the charge 9b induced on the center electrode 3 and the ground electrodes 4a, 4b of the traveling-wave electrode.
However, as can be understood from FIG. 21, since this electric line of force 7b is generated at random, the electric line of force 7a generated by the voltage applied between the center electrode 3 and the ground electrodes 4a, 4b for operating the optical modulation device is eliminated at random.
Therefore, the optical modulation efficiency of the optical modulation device widely varies, due to changes in temperature.
The change of this optical modulation efficiency occurs as a phenomenon of an operating point shift.
This operating point shift due to the temperature is called thermal drift.
In order to solve the disadvantages of the aforementioned optical modulation device, there has been proposed an optical modulation device having a sectional shape shown in FIG. 22 (refer to Jpn. Pat. Appln. KOKOKU Publication No. 4-22485).
In addition, in FIG. 22, the diagram is shown in a somewhat enlarged manner in the vertical direction in order to explain the operations.
Further, in the optical modulation device shown in FIG. 22, like reference numerals are denoted to like parts identical to those in the optical modulation device shown in FIG. 19.
Furthermore, a perspective view showing the entire structure of the optical modulation device shown in FIG. 22 is substantially identical to the perspective view of the optical modulation device shown in FIG. 18.
In the optical modulation device shown in FIG. 22, a conductive film 6 is formed on the upper side of the buffer layer 5, and the center electrode 3 and the ground electrodes 4a, 4b are formed on the upper side of the conductive film 6.
In other words, as shown in FIG. 22, a charge 9c having a polarity opposite the polarity of the charge 9a induced on the LN substrate 1 due to the pyroelectric effect of the LN substrate 1 is induced on the conductive film 6 in contact with the center electrode 3 and the ground electrodes 4a, 4b. 
As a result, an electric line of force 7c between the charge 9a induced on the LN substrate 1 and the charge 9c induced on the conductive film 6 becomes uniform as shown in FIG. 22 so that a random electric line of force is not across the optical waveguides 2a and 2b. 
This is to say, the changes of the refractive index caused by the charge 9a induced by the pyroelectric effect become the same in the two optical waveguides 2a and 2b. 
Thereby, in the optical modulation device shown in FIG. 22, the phase difference of the lights being guided through the two optical waveguides 2a and 2b is caused by only the voltage externally applied, which enables this device to function as an optical modulation device.
In addition, an Si film having the thickness of about 100 nm is employed as the conductive film 6.
However, even an optical modulation device employing the conductive film 6 in this manner still has important problems to be solved.
That is, as is well known, the control of the electric conductivity of the conductive film 6 is very difficult, which means that the conductivity thereof easily varies, in the order of 2 to 3 decimal places, due to impurities in the film.
When the conductivity of the conductive film 6 is too low, the device closely resembles an optical modulation device which does not employ a conductive film 6 shown in FIG. 19, meaning that the problems which this optical modulation device has occurs.
On the contrary, when the conductivity of the conductive film 6 is too high, the center electrode 3 and the ground electrodes 4a, 4b enter an electrically conductive state.
As a result, the characteristic impedance of the traveling-wave electrode formed of the center electrode 3 and the ground electrodes 4a, 4b is very lowered, thus the electric characteristics and high frequency characteristics are deteriorated, or a large current flows between the center electrode 3 and the ground electrodes 4a, 4b, so that the device itself is destroyed.
In this manner, it is remarkably difficult to form a conductive film 6 having an appropriate conductivity with excellent reproducibility, and the optical modulation device employing the conductive film 6 shown in FIG. 22 has a large problem in the reproducibility in the manufacture thereof.
In order to solve the disadvantages of the aforementioned optical modulation device, there has been further proposed an optical modulation device having a sectional shape shown in FIG. 23 (Japanese Patent No. 2873203).
In the optical modulation device shown in FIG. 23, like reference numerals are denoted to like parts identical to those in the optical modulation device shown in FIG. 19.
In the optical modulation device shown in FIG. 23, the width of the conductive film 6a is set to be finite so that the conductive film 6a is made to contact only the center electrode 3.
Further, in the optical modulation device shown in FIG. 23, a configuration is employed in which a gap 10, having a width G, is provided between the ground electrodes 4a, 4b at both sides, and a conductive film 6a, so that the ground electrodes 4a, 4b and the conductive film 6a do not contact.
Next, operations of the optical modulation device in which the width of the conductive film 6a is restricted in this manner are described.
With respect to the electric field applied to the optical waveguide 2a positioned below the center electrode 3, an electric line of force 7d caused by the uniform electric field distribution can be obtained by the same principles as the optical modulation device shown in FIG. 22 other than the influence of the electric field from the charge 9a induced by the pyroelectric effect in an area A and an area B in FIG. 23 due to the presence of the conductive film 6a. 
On the other hand, with respect to the ground electrodes 4a and 4b, the ground electrodes 4a and 4b are not in contact with the conductive film 6a different from the optical modulation device shown in FIG. 22.
Therefore, with respect to the optical waveguide 2b positioned in the area B in FIG. 23, it is predicted that the problem similar to that of the optical modulation device shown in FIG. 19 occurs.
In order to avoid that, the following devisal is made.
The thickness of the traveling waveguide of the center electrode 3 and the ground electrodes 4a, 4b is set to be as thick as several μm or more.
In other words, since the traveling-wave electrode made of metal (Au is generally used as a material, but various other meals such as aluminum, copper, and the like can be employed) and the LN substrate 1 of dielectrics have different thermal expansion coefficients, a temperature-dependent stress is generated in the LN substrate 1.
The internal stress due to plating becomes larger to the extent where a bow occurs in the LN substrate 1 when plating is performed to several μm or more.
The internal electric field occurs due to the photoelastic effect caused by this internal stress.
On the other hand, as described in the optical modulation device shown in FIG. 19, the charge 9a occurs in the surface of the LN substrate 1 by the pyroelectric effect when the temperature is changed such that the internal electric field occurs.
In the optical modulation device shown in FIG. 23, the gap 10 having the width G is provided between the conductive film 6a and the ground electrodes 4a, 4b at both sides and the width G is defined by photolithography. Therefore, the internal electric field by the photoelastic effect caused by the internal stress and the internal electric field by the pyroelectric effect caused by the temperature change are eliminated.
Accordingly, in the state where the characteristic impedance of the traveling-wave electrode formed of the center electrode 3 and the ground electrodes 4a, 4b is maintained in a constant state, it is prevented that the electric line of force 7a due to the voltage applied between the center electrode 3 and the ground electrodes 4a, 4b for operating the optical modulation device is eliminated at random.
However, even an optical modulation device in which the width of the conductive film 6a is restricted shown in FIG. 23 has the following further problem to be solved.
In other words, as can be easily assumed from the above description, it is not easy to appropriately eliminate the two internal electric fields described above generated in the LN substrate 1.
Further, when it cannot be realized, a random electric field is applied in the optical waveguide 2b positioned below the ground electrode 4b as with the optical modulation device shown in FIG. 19.
Particularly, the influence of the random electric field from the area A is large.
In addition, the random electric field from the area B particularly acts on the optical waveguide 2a below the center electrode 3.
As a result, the operating point in the optical modulation of the optical modulation device is largely deviates according to the temperature.
The traveling-wave electrode formed of the center electrode 3 and the ground electrodes 4a, 4b is generally formed by electrolytic plating method, but since a temperature variation or a current variation is present in the electrolytic plating solution, even when the same current is flowed at the same solution temperature at the time of plating, a slight variation occurs in the particle of the grown plating or the thickness of the electrode every run-to-run of the plating step.
Therefore, the internal stress caused by the formed plating is different in every plating.
Furthermore, in the electrolytic plating solution, a slight variation is present in the particles of the plating or the thickness of the electrode even in the same wafer, because of the fact that a variation is present in the current or the solution temperature between the plating electrode and the wafer, and the convection of the electrolytic plating solution, so that the internal stress is different in every chip.
As a result, it is difficult to determine the appropriate width G of the gap 10 for eliminating the internal electric field caused by the photoelastic effect due to the thermal stress and the internal electric field caused by the pyroelectric effect due to the temperature change.
Additionally, since the gap 10 is realized by photolithography, it is required to be performed before plating the traveling-wave electrode to 10 μm or more.
That is, the gap 10 is required to be formed at the stage where each modulation device including the LN substrate 1 is cut out from the wafer, namely in the wafer stage.
As a result, it has to be performed before measuring the thermal drift characteristics of each modulation device, and the determination of the width G of the gap 10 is difficult and is not necessarily appropriately performed with respect to each chip so that a yield of the optical modulation device including the LN substrate 1 is restricted.
Moreover, as described above, the effect obtained by eliminating the two electric fields described above generated in the LN substrate 1 is limited, and therefore the thickness of the plating of the traveling-wave electrode is limited to about 10 to 20 μm.
However, in the actual optical modulation device, in order to achieve a velocity matching between a microwave and a light required for broadbandizing of the optical modulation, sometimes the traveling-wave having the thickness of 25 μm to 30 μm or more may be required.
In this case, the optical modulation device having a mechanism for eliminating the two internal electric fields shown in FIG. 23 cannot be used and application has limitation.